In general, fuel cells are broadly classified into alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), direct methanol fuel cells (DMFCs) and polymer electrolyte membrane fuel cells (PEMFCs) by the kind of electrolyte that they employ.
Of these fuel cells, polymer fuel cells and direct methanol fuel cells use polymers as electrolytes to avoid the risk of corrosion by the electrolytes or evaporation of the electrolytes. Further, polymer fuel cells and direct methanol fuel cells provide much better output characteristics due to their higher current density per unit area and require lower operating temperatures than the other kinds of fuel cells. Based on these advantages, polymer fuel cells and direct methanol fuel cells are actively being developed for a variety of applications, including transportable power sources for automotive vehicles, distributed power sources (on-site) for houses and public buildings, and small power sources for electronic devices, in the United States, Japan and European countries. An ion conductive polymer electrolyte membrane is the most critical constituent element in determining the performance and price of a polymer electrolyte fuel cell or a direct methanol fuel cell.
Polymer electrolyte membranes that are currently in use include perfluorosulfonate ionomer membranes sold under the trademarks Nafion (DuPont), Flemion (Asahi Glass), Aciplex (Asahi Chemical) and Dow XUS (Dow Chemical). However, the high price of the commercial membrane products acts as an obstacle in the practical use of polymer fuel cells as power sources for electricity generation.
In view of the economic burden of the membrane products, research is being actively conducted on relatively inexpensive hydrocarbon polymers (such as polyether ether ketone, polysulfone and polyimide) that can be used in various commercial applications. Each of the hydrocarbon polymers undergoes sulfonation to give an ion conductive polymer, which is then cast into an electrolyte membrane for a fuel cell.
The greatest disadvantages of the hydrocarbon polymers are poor resistance to oxidation and reduction and thermal/mechanical instability. Another disadvantage of the hydrocarbon polymers is poor adhesion to electrodes resulting from excessive swelling in the production of membrane electrode assemblies (MEAs). In an attempt to overcome the above disadvantages, a method has been proposed in which a perfluorinated polymer or a hydrocarbon polymer is impregnated into the pores of a porous support (e.g., Teflon) having excellent mechanical and thermal properties and good oxidation resistance to produce a composite membrane. A representative composite membrane produced by the method is sold under the trade name Gore-select by W.L. Gore & Associates, which has a small thickness of 20 to 40 μm and exhibits superior mechanical and electrochemical properties.
Taking advantage of the excellent characteristics of the composite membrane, various methods have been proposed to produce electrolyte membranes for fuel cells with better performance that can replace Nafion. According to an exemplary method for producing an electrolyte composite membrane, styrene as a hydrocarbon monomer and divinylbenzene as a crosslinking agent are impregnated into a porous support selected from Teflon, polyethylene (PE) and polyvinylidene difluoride (PVDF), followed by crosslinking and sulfonation. According to another exemplary method for producing an electrolyte membrane, acrylsulfonic acid as a monomer and a water-soluble crosslinking agent are impregnated into a suitable porous support, followed by crosslinking.
The electrolyte membrane composed of polystyrene crosslinked with divinylbenzene tends to be fragile in a dry state due to its increased brittleness. Therefore, the electrolyte membrane suffers from mechanical instability when it is intended to form the electrolyte membrane into a thin film or a composite membrane for mass production and to process the electrolyte membrane into an electrode. Further, it is known that the electrolyte membrane composed of crosslinked polyacrylsulfonic acid and filled in the pores of a porous support is not put to practical use in various applications due to its drawbacks, such as poor durability.